The present invention relates to image setting apparatus for providing high quality, two-dimensional images on photosensitive material. More particularly, the present invention relates to a circuit for controlling the modulation of an electro-magnetic radiation scanning beam in apparatus of this type.
Text and graphics image setting apparatus of the type to which the present invention relates should be capable of providing two-dimensional image resolutions of up to 2400 dots per inch (dpi). Such a resolution may be achieved by scanning the surface of the photosensitive material with the focused spot of a modulated beam of electromagnetic radiation (hereinafter "EMR") such as infrared light. The EMR beam, which may be produced by a laser, is repeatedly scanned in a linear direction across the photosensitive material to provide one dimension of the two-dimensional image. The other, perpendicular image dimension is produced by repeatedly moving the photosensitive material, between or during each beam scan, in a direction perpendicular to the aforesaid beam scanning direction.
In order to obtain a uniform resolution of 2400 dpi, the scanning beam must be electronically modulated to achieve this resolution with the given scanning rate or speed of the beam spot along the imaging line. The photosensitive material must also be incremented with this resolution from scan line to scan line.
The present invention relates to a circuit which is capable of accurately controlling the modulation of the scanning EMR beam so as to achieve this, or any other, desired resolution in such a way that the resolution will be uniform.
Image setting apparatus of the aforementioned type typically includes a scan lens in the path of the scanning beam that maintains the beam spot substantially in focus on the photosensitive material at all positions along the imaging line. Such a lens, which is called an "f .theta." lens, and which may be made up of a plurality of individual lens components, operates in accordance with the principles described in the U.S. Pat. No. 3,687,075 issued Aug. 29, 1972 to Seymour Rosin. Specifically, the scan lens is constructed and arranged to focus the beam in accordance with the relationship Y'=f .theta., where Y' is the instantaneous distance of the beam spot from the intersection of the axis of the scan lens and the image plane, f is the effective focal length of the lens and .theta. is the instantaneous angle between the scanning beam and the optical axis of the scan lens. The scan lens thus focuses the beam on the photosensitive surface while compensating for any variations in image focus introduced by deviations of the beam from the optical axis.
In image setting apparatus of this type, the scanning device for converting the EMR beam into a scanning beam is normally a mechanically rotating mirror element having one or more reflecting facets. With such an arrangement, the scanning beam reflected from the scanning mirror rotates about the point of reflection with a substantially constant angular velocity. Since this beam impinges on a planar surface, in the absence of a focusing lens, an (unfocused) beam spot would not move with a constant speed along the imaging line. In particular, the beam spot would move at greater speed at points along the imaging line that are at some distance from the optical axis than it would at those points along the imaging line immediately adjacent to the optical axis.
The f .theta. scan lens may be designed to somewhat compensate for variations in speed of the focused beam spot along the imaging line; however, no lens can completely or even substantially eliminate such variations in scanning speed. Accordingly, when the beam is modulated with a constant frequency in accordance with the text and/or graphics information to be layed down on the photosensitive surface, these variations in beam scanning speed result in proportionate variations in the image resolution, causing concomitant distortion of the text and/or graphics image.
It is therefore a primary object of the present invention to provide means for maintaining a uniform resolution in an image setting apparatus notwithstanding variations in the scanning speed of the beam spot along the imaging line.